It only makes sense in the Matt Williams world to leave your starter in to face the batter who hit a homer against him when you have a million relievers in the bullpen. He would never go to one of those reliever to pitch the 6th and then ph for him.
...and then the starter doesn't even finish the inning so he has to bring in a reliever to pitch anyway...and then gets pinch hit for.
It only makes sense to Matt.